1. Field of the Invention
This invention relates to the production of herbicidal esters of 3,5-dibromo-4-hydroxybenzonitrile and in particular to a novel process for producing such esters of high purity and in near quantitative yield.
2. Description of the Prior Art
The use of 3,5-dibromo-4-hydroxybenzonitrile as a herbicide is well established. Certain esters of 3,5-dibromo-4-hydroxybenzonitrile, especially the octanoate, have been used extensively as herbicides in the control of broad-leafed weeds, particularly in crop growing areas. These bromobenzonitrile derivatives have great economic value since they have been shown to be highly effective when applied to the growth of crops such as cereals, grasses, sugar cane, legumes, flax, linen, and various vegetables. These herbicides completely control undesirable vegetation at relatively low application rates without harming the crops and thus allow the crops to grow freely. The use of the 3,5-dibromo-4-n-octanoyloxybenzonitriles and their diiodo analogs as herbicides for treatment of broad-leafed weeds is disclosed in Haywood, et al., U.S. Pat. No. 3,592,626, granted July 13, 1971.
Although the esters have thus been shown to be highly effective, they have heretofore been produced only by incurring productivity penalties, which are aggravated by the expensive and cumbersome purifications that have heretofore been required.
3,5-Dibromo-4-hydroxybenzonitrile was first prepared by Auwers and Reis (1896) by a complex four step procedure starting with 4-hydroxybenzaldehyde, an uneconomical procedure involving bromination, formation of the aldoxime, dehydration with concomitant acetate formation and hydrolytic removal of the acetate.
Much later, E. Muller, et al., Chem. Ber. 92, 2278 (1959), described a procedure for bromination of 4-hydroxybenzonitrile with elemental bromine in methanolic acetic acid. The product produced by the Muller, et al., process was recovered by contacting the resulting bromination mixture with aqueous methanolic sodium hydrogen sulfite.
Luckenbaugh, U.S. Pat. No. 3,349,111, describes the production of 3,5-dibromo-4-hydroxybenzonitrile or its sodium salt by carrying out bromination with elemental bromine in an aqueous suspension, especially aqueous caustic, followed by a chlorine sparge.
French Pat. No. 1,375,311 describes bromination of hydroxybenzonitrile in acetic acid, obtaining 3,5-dibromo-4-hydroxybenzonitrile in 60% yield. The patent also suggests that by bromination with aqueous sodium hypobromite the product may be obtained in 78% yield.
While the production of 3,5-dibromo-4-hydroxybenzonitrile itself has only been obtained by difficult procedures or with poor yield and overall economics, the methods heretofore known for obtaining the herbicidally more active esters have been fraught with even greater difficulty.
The two principal methods used to obtain esters of 3,5-dibromo-4-hydroxybenzonitrile have involved either reaction with an organic anhydride in the presence of a condensing agent or reaction of bromobenzonitrile with an acid halide in the presence of at least stoichiometric quantities of a base such a pyridine or a quarternary ammonium salt.
Thus, Hart, et al., U.S. Pat. No. 3,397,054, describes the production of alkanoyl derivatives of halo-substituted 4-hydroxybenzonitriles by reaction of 3,5-dibromo-4-hydroxybenzonitrile with the appropriate acid anhydride in pyridine as a solvent. The deficiencies of this procedure include the fact that only one-half of the acid anhydride is incorporated into the product, while the remaining acid must be removed as an impurity. Also, the use of pyridine as a solvent presents recovery difficulties and problems of toxicity and cost.
French Pat. No. 1,375,311 describes the production of 2,6-dibromo-4-cyanophenyl octanoate by reacting purified 3,5-dibromo-4-hydroxybenzonitrile with octanoyl chloride in excess pyridine, again presenting serious problems of toxicity, cost and recovery.
As a result of the foregoing disadvantages of the prior art acid anhydride and acid halide processes, Goldstick, U.S. Pat. No. 3,671,556, suggests that herbicidal esters of 3,5-dihalo-4-hydroxybenzonitriles could be obtained by slow direct addition of dry, solid dihalohydroxybenzonitrile to hot liquid acid halide. However, even this method presents difficulties, including plant scale solids handling problems, foaming, long reaction cycles, and the necessity for isolating and recovering purified dihalohydroxybenzonitrile intermediates.
Accordingly, a primary object of this invention is to obtain a process for simply and economically converting 4-hydroxybenzonitrile to esters of 3,5-dibromo-4-hydroxybenzonitrile of exceptionally high purity and in near quantitative yield.
A further object is to provide a process of the character described in which the intermediate 3,5-dibromo-4-hydroxybenzonitrile is reacted in situ to produce the resultant ester without being recovered or purified.
A still further object is to obtain a process of the character described in which large quantities of hazardous and expensive materials such as pyridine need not be employed.
A still further object is to provide a process of the character described in which catalytic amounts of pyridine or pyridine hydrohalides are employed with pyridine hydrohalides being recovered and recycled.